1. Field of the Invention
The present invention relates to a metal separator for a solid oxide regenerative fuel cell coated with a conductive spinel oxide film. More specifically, the present invention relates to a metal separator for a solid oxide regenerative fuel cell coated with a conductive spinel oxide film in which yttrium is added to a manganese-cobalt spinel oxide to suppress growth of an insulating oxide film on the surface of the metal separator and volatilization of metal, and a method for producing the metal separator coated with the conductive spinel oxide film.
2. Description of the Related Art
A unitized regenerative fuel cell (URFC) is an energy conversion and storage system that can work both as a fuel cell and as a water electrolyzer. Since the commercialization of fuel cells, unitized regenerative fuel cells (URFCs) have attracted considerable attention as next-generation fuel cells and have been emerging as the most promising topics playing a leading role in fuel cell research.
A unitized regenerative fuel cell (URFC) operates both in a fuel cell mode and in an electrolyzer mode. Electrical energy is produced in the fuel cell mode, which is the same as the operation mode of an existing fuel cell. In the electrolyzer mode, the URFC supplies electrical energy to electrolyze water as a byproduct of the fuel cell reaction into hydrogen and oxygen. The regenerated hydrogen and oxygen are supplied to the fuel cell to produce electrical energy. This is the very core of URFC technology. Due to the use of regenerated hydrogen, the utilization efficiency of hydrogen energy of the URFC is estimated to be significantly higher by about 10-15% than that of an existing fuel cell.
A solid oxide regenerative fuel cell (SORFC) using a ceramic membrane as an electrolyte is considered the most efficient type of URFC. In the solid oxide regenerative fuel cell, a solid oxide fuel cell (SOFC) for clean energy conversion with high efficiency is integrated with a solid oxide electrolysis cell (SOEC) for hydrogen production, which operates in the reverse mode to the SOEC. The solid oxide regenerative fuel cell is capable of high capacity supply power and hydrogen production/storage/utilization. Due to these advantages, the solid oxide regenerative fuel cell is suitable for stable supply of non-uniform low-quality power of renewable energy on demand. That is, the solid oxide regenerative fuel cell can use excess power of renewable energy to produce and store hydrogen when power demand is low, and can use the stored hydrogen as a fuel to supply power when power demand is high. In comparison with a regenerative fuel cell, the solid oxide regenerative fuel cell operating at a relatively high temperature of 800° C. is advantageous from thermodynamic and kinetic aspects and possesses relatively high efficiency and performance. Under such circumstances, reaction problems between materials, development of new materials, improvement of electrode characteristics, stack fabrication, and evaluation of operating tests are emerging as key issues in research and development.
However, performance deterioration of a variety of elements when exposed to high temperatures is the most serious obstacle to the commercialization of solid oxide regenerative fuel cells. Particularly, metal separators tend to be more susceptible to oxidation than ceramic materials. This tendency leads to poor performance of stacks, which is a very serious situation. In an SORFC stack, a metal separator electrically connects a cathode of one cell to an anode of another cell and serves and separates the two electrodes to prevent mixing of air and hydrogen. An Fe—Cr alloy as a main material for the metal separator is highly resistant to heat and oxidation. However, when the Fe—Cr alloy is exposed to an oxidizing atmosphere, i.e. air or water vapor, at a temperature as high as 800° C. at which a solid oxide regenerative fuel cell is operated, for a long time, an insulating oxide film with high electrical resistance grows on the metal surface, resulting in increased electrical resistance and performance deterioration of the stack.
When the Fe—Cr alloy comes into contact with oxygen at high temperature, a highly volatile chromium oxide (CrOX) is formed and the chromium (Cr) atoms are volatilized from the metal and deposited on the surface of electrodes, resulting in a reduction in the number of reaction sites of the electrodes and performance deterioration of the electrodes.
In an attempt to prevent growth of an insulating oxide film and minimize performance deterioration of a solid oxide regenerative fuel cell stack by chromium poisoning, research is being conducted on a technique for coating a conductive oxide film on the surface of a metal separator to prevent direct contact between the metal separator and atmospheric oxygen. However, current techniques for coating conductive oxide films suffer from limitations because the conductive oxide films are not dense enough to block gas permeation and are very difficult to form on metals.